1. Field of the Invention
The present invention relates to an elastic wave filter, for instance, a SAW (SAW: Surface Acoustic Wave) filter.
2. Description of the Related Art
A SAW device makes good use of surface acoustic waves and by arranging electrode fingers called an inter-digital transducer (IDT) above a piezoelectric substrate, and performing an electromechanical interconversion between an electric signal and an elastic wave, so as to have frequency selection (band filter) characteristics. The surface acoustic wave (SAW) filter, one of the SAW devices is used as a band-pass filter for various communication devices such as a cellular phone or the like, for which sophistication in performance and miniaturization have been proceeding. In recent years, with an advance of speeding up and enlarging capacity of wireless data communication, demands for a smaller insertion loss (attenuation amount of output power against input power), excellent filter characteristics in frequency selectivity, a broader band width, an increase in flatness, and in realizing miniaturization have been growing. In order to satisfy these demands, it is advantageous to use, for instance, a tapered IDT filter.
Such a filter 100 is provided with an input side tapered IDT electrode 102 and an output side tapered IDT electrode 103, which are tapered electrodes formed above a piezoelectric substrate 101, as shown, for instance, in FIG. 13, and is configured such that the elastic waves propagate from the input side tapered IDT electrode 102 side toward the output side tapered IDT electrode 103 side. A shield 104 to suppress coupling between the electrodes 102 and 103 is provided between these electrodes 102 and 103, and the shield 104 is formed as a square-shaped planar metal film (so-called solid film).
The respective electrodes 102 and 103 are composed of two parallel bus bars 105 having a plurality of electrode fingers 106, and are configured as a SPLIT electrode, by facing the electrode fingers 106 each other, connected to the bus bars 105, and by making up the electrode fingers 106 be sets of, for instance, pairs and extending alternately from the opposite side to be in a shape of comb teeth in the respective electrodes 102 and 103.
In each of the electrodes 102 and 103, the electrode finger 106 is formed so that the widths of the electrode fingers 106 become uniform along the direction of propagation of the elastic wave and the spaces between the electrode fingers 106 and 106 also become uniform. An arrangement pattern composed of the width of the electrode finger 106 and the space between the electrode fingers 106 and 106 is set in a manner that a cycle unit λ of a certain length is repeated. In this instance, one cycle unit λ is composed by four electrode fingers 106 and the space areas between these electrode fingers 106.
Accordingly, in this filter 100, the elastic wave having a wavelength the same as the cycle unit λ is to be propagated from the input side tapered IDT electrode 102 toward the output side tapered IDT electrode 103.
The length of the cycle unit λ is set in a manner that it is gradually widened from one bus bar 105 toward the other bus bar 105 in the direction perpendicular to the direction of propagation of the elastic wave, in other words, the width of the electrode finger 106 and the space between electrode fingers 106 and 106 are gradually widened respectively.
By configuring the tapered IDT by gradually widening the arrangement pattern of the electrode fingers 106 in the manner described above, the elastic waves having a range from a high frequency in which the cycle unit λ corresponds to a narrow area to a low frequency in which the cycle unit λ corresponds to a wide area, are propagated in the filter 100, which results in realization of widening band width of the filter 100.
In the meantime, when the arrangement pattern of the electrode finger 106 is gradually widened, an inclination angle θ formed between the electrode finger 106 and the direction of propagation of the elastic wave is getting increased. Furthermore, when a difference in length of the cycle unit λ between the cycle unit λ corresponding to the narrow area and the cycle unit λ corresponding to the wide area is taken large for the purpose of further widening the band width of propagating frequency in the filter 100, the inclination is further increased.
By the way, a difference in a state of propagation (propagation speed) of the elastic wave above the piezoelectric substrate 101 is created between a part where the input side tapered IDT electrode 102 and the output side tapered IDT electrode 103 exist and a part where no existence of the input side tapered IDT electrode 102 and the output side tapered IDT electrode 103. Then, the elastic wave is refracted when radiated from the end of the input side tapered IDT electrode 102 on the output side tapered IDT electrode 103 side. Accordingly, when the inclination angle θ of the electrode finger 106 is increased thus, the elastic wave propagated from the input side IDT electrode 102 launches into the output side tapered IDT electrode 103, largely shifted from a track being a propagation path where the cycle unit λ corresponding to its wavelength is formed.
When two kinds of the elastic waves, for instance, Tr 1 (low frequency side) and Tr 2 (high frequency side) are radiated from tracks where cycle units λ corresponding to respective frequencies in the input side tapered IDT electrode 102 are formed, toward the output side tapered IDT electrode 103 side in such a filter 100, FIG. 14 shows an energy distribution (right hatching in the drawing) in the case of just receiving the elastic wave transmitted from the respective tracks without shifting from the track and an energy distribution (left hatching in the drawing) in the case of receiving with shifting from the track. From FIG. 14, it is found that the energy received at the output side tapered IDT electrode 103 is reduced due to refraction in all tracks in either side of the low frequency side and the high frequency side. When compared the extents of the energy reduction, since the high frequency side Tr 2 is longer than the low frequency side Tr 1 as for a distance L between the input side tapered IDT electrode 102 and the output side tapered IDT electrode 103, regarding to dTr which shows degree of the track shifting, dTr 2 is larger than dTr 1, which results in larger attenuation amount in energy on the high frequency side.
Further, diffraction of the elastic wave radiated from the end of the input side tapered IDT electrode 102 occurs, which leads to loss based on the diffraction in the energy propagation between the input side tapered IDT electrode 102 and the out put side tapered IDT electrode 103. Then also, the longer the distance L between the input side tapered IDT electrode 102 and the output side tapered IDT electrode 103, the greater the diffraction loss.
From these circumstances, the energy loss due to refraction or diffraction gives large influence to the high frequency side characteristics having a long propagation distance L, and therefore, attenuation characteristics on the high frequency side are deteriorated as shown by “B” in FIG. 15.
Further, shown as Tr 0 (the side having a frequency lower than Tr 1) in FIG. 14, there is an elastic wave propagating to an area out of the area in which the output side tapered IDT electrode 103 is formed due to the refraction among the elastic waves radiated from the input side tapered IDT electrode 102. Since such an elastic wave is not received at the output side tapered IDT electrode 103, the attenuation characteristics is deteriorated even on the low frequency side.
In the meantime, in such a filter 100, in order to enhance the selectivity, in other words, in order to give a steep leading edge of a pass band in a frequency characteristic chart, a method of increasing a logarithm of the electrode finger 106 is adopted in general. Accordingly, the inclination angle θ of the electrode finger 106 is increased further, and deterioration of the above-described attenuation characteristics is more remarkable.
Concluding above, there are following problems.
Since a tapered IDT is inclined, the attenuation characteristics are deteriorated from a low frequency side to a high frequency side, and the extent of deterioration is great on a high frequency side in particular. When the filter 100 is made to have a higher frequency band, and when the logarithm of the electrode finger 106 is increased, the inclination angle θ increases, which makes such deterioration remarkable. In addition, when the logarithm of the electrode finger 106 is increased, there arises another problem of enlarging the size of the filter 100.
The above-described problems are found in Patent Document 1, but there is no suggestion on a technology to suppress refraction or diffraction of the elastic waves.
Patent Document 1
Japanese Patent Application Laid-open No. 2005-150918 ((0004), (0022) to (0025))